U.S. patent application number 15/748286 was filed with the patent office on 2018-08-02 for optical interferometer.
This patent application is currently assigned to HAMAMATSU PHOTONICS K.K.. The applicant listed for this patent is HAMAMATSU PHOTONICS K.K.. Invention is credited to Kohei KASAMORI, Kyosuke KOTANI, Ryosuke OKUMURA, Tomofumi SUZUKI, Yoshihisa WARASHINA.
Application Number | 20180217001 15/748286 |
Document ID | / |
Family ID | 55069297 |
Filed Date | 2018-08-02 |
United States Patent
Application |
20180217001 |
Kind Code |
A1 |
WARASHINA; Yoshihisa ; et
al. |
August 2, 2018 |
OPTICAL INTERFEROMETER
Abstract
An optical interferometer includes a branching-combining unit, a
first optical system, a second optical system, and a drive unit.
The branching-combining unit includes a branching surface, an
incident surface, a first output surface, a combining surface, and
a second output surface on an interface of a transparent member,
the branching surface partially reflects incident light and outputs
as first branched light, and transmits the rest of the incident
light into the interior as second branched light, the combining
surface partially combines the first branched light and the second
branched light to be output to the outside as first combined light,
and combines the rest of the first branched light and the second
branched light to be propagated into the interior as second
combined light, and the second output surface partially outputs the
second combined light to the outside.
Inventors: |
WARASHINA; Yoshihisa;
(Hamamatsu-shi, Shizuoka, JP) ; SUZUKI; Tomofumi;
(Hamamatsu-shi, Shizuoka, JP) ; KASAMORI; Kohei;
(Hamamatsu-shi, Shizuoka, JP) ; OKUMURA; Ryosuke;
(Hamamatsu-shi, Shizuoka, JP) ; KOTANI; Kyosuke;
(Hamamatsu-shi, Shizuoka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HAMAMATSU PHOTONICS K.K. |
Hamamatsu-shi, Shizuoka |
|
JP |
|
|
Assignee: |
HAMAMATSU PHOTONICS K.K.
Hamamatsu-shi, Shizuoka
JP
|
Family ID: |
55069297 |
Appl. No.: |
15/748286 |
Filed: |
September 3, 2015 |
PCT Filed: |
September 3, 2015 |
PCT NO: |
PCT/JP2015/075105 |
371 Date: |
January 29, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01B 9/02058 20130101;
G01B 9/02059 20130101; G01B 9/02017 20130101; G01B 9/02022
20130101; G01B 9/02044 20130101; G01J 3/4532 20130101; G01J 3/45
20130101 |
International
Class: |
G01J 3/45 20060101
G01J003/45; G01B 9/02 20060101 G01B009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2015 |
JP |
2015-150546 |
Claims
1. An optical interferometer comprising a branching-combining unit;
a first optical system; a second optical system; and a drive unit,
wherein the branching-combining unit includes a branching surface,
an incident surface, a first output surface, a combining surface,
and a second output surface on an interface between the interior
and the exterior of a transparent member, the branching surface and
the combining surface are provided separately, the branching
surface partially reflects incident light entering from the outside
and outputs as first branched light, and transmits the rest of the
incident light into the interior as second branched light, the
incident surface transmits the first branched light entering from
the branching surface via the first optical system into the
interior, the first output surface outputs the second branched
light reaching from the branching surface through the interior to
the outside, the combining surface partially combines the first
branched light reaching from the incident surface through the
interior, and the second branched light entering from the first
output surface via the second optical system to be output to the
outside as first combined light, and combines the rest of the first
branched light and the second branched light to be propagated into
the interior as second combined light, and the second output
surface partially outputs the second combined light reaching from
the combining surface through the interior to the outside, and the
first optical system reflects is configured to reflect the first
branched light output from the branching surface by a mirror, and
directs direct the light to the incident surface, the second
optical system reflects is configured to reflect the second
branched light output from the first output surface by a mirror,
and directs direct the light to the combining surface, and the
drive unit is configured to move the mirror of the first optical
system or the second optical system to adjust an optical path
difference between the first branched light and the second branched
light from the branching surface to the combining surface.
2. The optical interferometer according to claim 1, further
comprising: a first light receiving element configured to receive
the first combined light output from the combining surface and
output a first current signal I.sub.1 according to an amount of the
received light; a second light receiving element configured to
receive the second combined light partially output from the second
output surface and output a second current signal I.sub.2 according
to an amount of the received light; and a current-voltage
conversion unit configured to output a voltage signal according to
a value (I.sub.1-I.sub.2) obtained by subtracting the second
current signal I.sub.2 from the first current signal I.sub.1.
3. The optical interferometer according to claim 1, wherein the
branching-combining unit further includes a third output surface on
the interface between the interior and the exterior of the
transparent member, and the third output surface outputs the rest
of the second combined light reflected by the second output surface
and reaching through the interior to the outside.
4. The optical interferometer according to claim 3, further
comprising: a first light receiving element configured to receive
the first combined light output from the combining surface and
output a first current signal I.sub.1 according to an amount of the
received light; a second light receiving element configured to
receive the second combined light partially output from the second
output surface and ouput a second current signal I.sub.2 according
to an amount of the received light; a third light receiving element
configured to receive the rest of the second combined light output
from the third output surface and output a third current signal
I.sub.3 according to an amount of the received light; and a
current-voltage conversion unit configured to output a voltage
signal according to a value (I.sub.1-I.sub.2-I.sub.3) obtained by
subtracting the second current signal I.sub.2 and the third current
signal I.sub.3 from the first current signal I.sub.1.
5. The optical interferometer according to claim 1, wherein the
branching-combining unit further includes a total reflection
surface on the interface between the interior and the exterior of
the transparent member, and the total reflection surface totally
reflects the second combined light reaching from the combining
surface through the interior and directs the light to the second
output surface.
6. The optical interferometer according to claim 1, wherein the
first optical system and the second optical system include an even
number of mirrors in total, and the optical interferometer is
configured to branch a light ray at each position in a beam
cross-section of the incident light on the branching surface, and
then combine the light rays at a common position in a beam
cross-section of the first combined light and the second combined
light on the combining surface.
7. The optical interferometer according to claim 1, wherein the
branching-combining unit, the first optical system, the second
optical system, and the drive unit are MEMS-based components.
8. The optical interferometer according to claim 1, wherein the
branching-combining unit is made of silicon.
9. The optical interferometer according to claim 1, wherein the
first branched light and the second branched light have the same
optical path length in the branching-combining unit.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical
interferometer.
BACKGROUND ART
[0002] An optical interferometer disclosed in Patent Document 1
uses a branching-combining unit made of, for example, silicon to
partially reflect incident light on one plane of the
branching-combining unit, and transmit the rest of the incident
light through the plane, to branch the light into first branched
light and second branched light, and combine the first branched
light and the second branched light and output as combined light.
That is, the optical interferometer commonly uses one plane of the
branching-combining unit as the branching surface for branching the
incident light into the first branched light and the second
branched light and the combining surface for combining the first
branched light and the second branched light to form the combined
light. Further, in the optical interferometer disclosed in this
document, wavelength dispersion occurs when one light of the first
branched light and the second branched light reciprocates in the
branching-combining unit, and to eliminate the wavelength
dispersion, the other light is made to reciprocate in a dispersion
compensating member.
[0003] An optical interferometer disclosed in Patent Document 2
uses a branching-combining unit made of, for example, silicon to
partially reflect incident light on a first principal surface of
the branching-combining unit, and transmit the rest of the incident
light through the surface, to branch the light into first branched
light and second branched light, and combine the first branched
light and the second branched light on a second principal surface
of the branching-combining unit and output as combined light. That
is, the optical interferometer uses different surfaces as the
branching surface (first principal surface) for branching the
incident light into the first branched light and the second
branched light and the combining surface (second principal surface)
for combining the first branched light and the second branched
light to form the combined light. The optical interferometer
disclosed in this document can decrease the wavelength dispersion,
because each of the first branched light and the second branched
light passes through the branching-combining unit only once.
CITATION LIST
Patent Literature
[0004] Patent Document 1: Japanese Patent Application Laid-Open
Publication No. 2013-504066
[0005] Patent Document 2: Japanese Patent Application Laid-Open
Publication No. H3-77029
SUMMARY OF INVENTION
Technical Problem
[0006] Inventors of the present invention have found that the
conventional optical interferometers, including those disclosed in
Patent Documents 1 and 2, have the following problem. That is, the
conventional optical interferometers have a low light use
efficiency, because only a small ratio of incident light is used in
the combined light. Generally, in the optical interferometer, light
loss which is inevitable in principle is generated on the branching
surface and the combining surface, and further, additional light
loss (hereinafter referred to as "excessive loss") is
generated.
[0007] The present invention has been made in order to solve the
above problem, and an object thereof is to provide an optical
interferometer capable of decreasing a ratio of excessive loss.
Solution to Problem
[0008] An optical interferometer according to the present invention
includes a branching-combining unit, a first optical system, a
second optical system, and a drive unit. The branching-combining
unit includes a branching surface, an incident surface, a first
output surface, a combining surface, and a second output surface on
an interface between the interior and the exterior of a transparent
member, the branching surface and the combining surface are
provided separately, the branching surface partially reflects
incident light entering from the outside and outputs as first
branched light, and transmits the rest of the incident light into
the interior as second branched light, the incident surface
transmits the first branched light entering from the branching
surface via the first optical system into the interior, the first
output surface outputs the second branched light reaching from the
branching surface through the interior to the outside, the
combining surface partially combines the first branched light
reaching from the incident surface through the interior, and the
second branched light entering from the first output surface via
the second optical system to be output to the outside as first
combined light, and combines the rest of the first branched light
and the second branched light to be propagated into the interior as
second combined light, and the second output surface partially
outputs the second combined light reaching from the combining
surface through the interior to the outside. The first optical
system reflects the first branched light output from the branching
surface by a mirror, and directs the light to the incident surface.
The second optical system reflects the second branched light output
from the first output surface by a mirror, and directs the light to
the combining surface. The drive unit moves the mirror of the first
optical system or the second optical system to adjust an optical
path difference between the first branched light and the second
branched light from the branching surface to the combining
surface.
Advantageous Effects of Invention
[0009] The present invention can provide an optical interferometer
that can decrease a ratio of excessive loss.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a diagram illustrating a configuration of an
optical interferometer 2A of a first comparative example.
[0011] FIG. 2 is a diagram illustrating a configuration of an
optical interferometer 2B of a second comparative example.
[0012] FIG. 3 shows an interferogram when white light enters the
optical interferometer.
[0013] FIG. 4 shows an interferogram when white light enters the
optical interferometer.
[0014] FIG. 5 is a diagram illustrating a configuration of an
optical interferometer 1A of a first embodiment.
[0015] FIG. 6 is a diagram illustrating a configuration of an
optical interferometer 2C of a third comparative example.
[0016] FIG. 7 is a diagram illustrating a configuration of an
optical interferometer 2D of a fourth comparative example.
[0017] FIG. 8 is a graph illustrating a relationship between a
refractive index of a material of a branching-combining unit 10 and
an average interference intensity peak.
[0018] FIG. 9 shows interferograms of combined light L.sub.3 and
combined light L.sub.41.
[0019] FIG. 10 shows a difference between the interferograms of the
combined light L.sub.3 and the combined light L.sub.41.
[0020] FIG. 11 is a circuit diagram of a detection unit 50 of the
optical interferometer 1A of the first embodiment.
[0021] FIG. 12 is a diagram illustrating a configuration of an
optical interferometer 1B of a second embodiment.
[0022] FIG. 13 is a circuit diagram of a detection unit 50 of the
optical interferometer 1B of the second embodiment.
[0023] FIG. 14 is a diagram illustrating a configuration of an
optical interferometer 1C of a third embodiment.
DESCRIPTION OF EMBODIMENTS
[0024] Hereinafter, embodiments for carrying out the present
invention will be described in detail with reference to the
accompanying drawings. In the description of the drawings, the same
elements will be denoted by the same reference signs, without
redundant description.
[0025] The present invention is not limited to these examples, and
the Claims, their equivalents, and all the changes within the scope
are intended as would fall within the scope of the present
invention.
[0026] Before describing optical interferometers of the
embodiments, optical interferometers of comparative examples to be
compared with the embodiments are described.
FIRST COMPARATIVE EXAMPLE
[0027] FIG. 1 is a diagram illustrating a configuration of an
optical interferometer 2A of a first comparative example. The
optical interferometer 2A includes a branching-combining unit 10, a
mirror 21, a mirror 31, and a dispersion compensating member
90.
[0028] The branching-combining unit 10 is made of, for example,
silicon and has a first principal surface 10a and a second
principal surface 10b which are parallel to each other. Incident
light L.sub.0 that enters from the outside to the first principal
surface 10a is partially reflected as first branched light
L.sub.11, and the rest of the incident light transmits into the
interior of the branching-combining unit 10 as second branched
light L.sub.21.
[0029] The first branched light L.sub.11 from the first principal
surface 10a passes through the interior of the dispersion
compensating member 90 and is reflected by the mirror 21. The first
branched light L.sub.12 reflected by the mirror 21 passes through
the interior of the dispersion compensating member 90 again, enters
the first principal surface 10a, and transmits into the interior of
the branching-combining unit 10.
[0030] The second branched light L.sub.21 from the first principal
surface 10a passes through the interior of the branching-combining
unit 10, transmits through the second principal surface 10b to be
output to the outside, and is reflected by the mirror 31. The
second branched light L.sub.22 reflected by the mirror 31 enters
the second principal surface 10b, transmits into the interior of
the branching-combining unit 10, passes through the interior of the
branching-combining unit 10 again, and is reflected by the first
principal surface 10a.
[0031] The first branched light L.sub.12 transmitted into the
interior of the branching-combining unit 10 on the first principal
surface 10a and the second branched light L.sub.22 reflected on the
first principal surface 10a are combined to form combined light
L.sub.3. The combined light L.sub.3 passes through the interior of
the branching-combining unit 10, and transmits through the second
principal surface 10b to be output to the outside. The combined
light L.sub.3 output to the outside is detected by a detection unit
50.
[0032] In the optical interferometer 2A, the first principal
surface 10a of the branching-combining unit 10 is used as both the
branching surface for branching the incident light L.sub.0 into the
first branched light L.sub.11 and the second branched light
L.sub.21, and the combining surface for combining the first
branched light L.sub.12 and the second branched light L.sub.22 into
the combined light L.sub.3.
[0033] For example, the position of the mirror 21 is fixed and the
mirror 31 is movable by a drive unit along the incident direction
of the second branched light L.sub.21. The drive unit can also be
configured by a MEMS-based component. Since the mirror 31 is
movable, the optical path difference between the first branched
light and the second branched light is adjustable.
[0034] In the optical interferometer 2A, the branching-combining
unit 10 and the dispersion compensating member 90 are made of the
same material (e.g., silicon). Further, the optical path length of
a section where the first branched light L.sub.11, L.sub.12
reciprocates in the dispersion compensating member 90 is set to be
equal to the optical path length of a section where the second
branched light L.sub.21, L.sub.22 reciprocates in the
branching-combining unit 10. This eliminates the problem of
wavelength dispersion and decreases wavelength dependency of the
optical path difference between the first branched light and the
second branched light.
[0035] When the optical interferometer 2A is used in an infrared
spectroscopic device, such as FTIR (Fourier Transform Infrared
Spectroscopy) in which the branching-combining unit 10 is made of
quartz glass, the transmission characteristic of quartz glass
allows the optical interferometer 2A to be used only in the
near-infrared region. Further, quartz glass does not have a high
refractive index, so that a dielectric multi-layer film needs to be
formed on the surface of the quartz glass for adjusting a branching
ratio. Further, a different branching-combining unit 10 made of a
material appropriate for each wavelength region can be used,
however, in that case, it is necessary to change the
branching-combining unit 10 according to the wavelength region and
perform optical adjustment, which is inconvenient. Further, in the
optical interferometer fabricated by the MEMS technique, it is not
possible to freely select the branching-combining unit 10.
[0036] To prevent the above problem, it is preferable to use a
semiconductor material such as silicon for the branching-combining
unit 10. Silicon has a refractive index of around 3.5 in the
near-infrared region, and when near-infrared light enters
vertically on an interface between silicon and air, the reflectance
of Fresnel reflection that occurs due to a difference in refractive
index between both media is about 30%. That is, the interface of
silicon itself can be used as a branching surface having an
appropriate branching ratio, without forming a dielectric
multi-layer film for adjusting the branching ratio on the surface
of the branching-combining unit 10 made of silicon. In addition,
silicon can function as a medium which is transparent up to the
far-infrared region (more than several tens of micrometers), so
that there is no need to strictly control the thickness. Here,
silicon has a larger absorption characteristic in the wavelength
region of not longer than 1.1 .mu.m, so that silicon can be used in
an infrared region not shorter than 1.1 .mu.m. In the near-infrared
region not longer than 1 .mu.m, germanium can be used. Although
germanium can be used up to about 5 .mu.m in the mid-infrared
region, but has a refractive index of about 4.0, so that germanium
achieves a better branching characteristic than that of
silicon.
[0037] Thus, the branching-combining unit 10 made of a
semiconductor material such as silicon or germanium can branch and
combine broadband light. On the other hand, however, in the optical
interferometer 2A, excessive loss of light is generated, in
addition to the light loss which is inevitable in principle in
branching and combining. This problem is described below.
[0038] When the first branched light L.sub.12 reflected by the
mirror 21 enters the first principal surface 10a, transmitted light
transmitted into the interior and reflected light are generated,
the transmitted light contributes to interference, and the
reflected light does not contribute to interference and becomes a
loss. However, this is an inevitable loss in combining and the
reflected light becomes the loss which is inevitable in
principle.
[0039] Similarly, when the second branched light L.sub.22 reflected
by the mirror 31 enters the first principal surface 10a, reflected
light and transmitted light are generated, the reflected light
contributes to interference, and the transmitted light does not
contribute to interference and becomes a loss. However, this is
also an inevitable loss in combining and the transmitted light
becomes the loss which is inevitable in principle.
[0040] However, the reflected light that is generated on the second
principal surface 10b when the second branched light L.sub.21
transmits through the second principal surface 10b to the mirror 31
is not the loss which occurs inevitably in principle, and is the
excessive loss. Further, the reflected light that is generated on
the second principal surface 10b when the second branched light
L.sub.22 reflected by the mirror 31 transmits through the second
principal surface 10b to the first principal surface 10a is not the
loss which occurs inevitably in principle, and is the excessive
loss. In addition, the reflected light that is generated on the
second principal surface 10b when the combined light L.sub.3
transmits through the second principal surface 10b and is output to
the outside is not the loss which occurs inevitably in principle,
and is the excessive loss.
[0041] Here, assume that a power reflectance in Fresnel reflection
of light on each of the first principal surface 10a and the second
principal surface 10b of the branching-combining unit 10 is R. At
this time, a power transmittance is 1-R, and a branching ratio is
R:(1-R). Generally, when light enters an interface between two
media having different refractive indexes, the reflectance and
transmittance vary depending on a polarization direction and an
incident angle of the incident light.
[0042] In the optical system of the optical interferometer 2A
illustrated in FIG. 1, S-polarization components of the incident
light L.sub.0 that enter the first principal surface 10a remain as
the S-polarization components, and P-polarization components of the
incident light L.sub.0 remain as the P-polarization components.
Further, when the first principal surface 10a and the second
principal surface 10b are parallel to each other, and the incident
angle of the incident light L.sub.0 to the first principal surface
10a is 45 degrees, then the light that enters the first principal
surface 10a or the second principal surface 10b from the outside
has the incident angle of 45 degrees, and the light that is output
from the first principal surface 10a or the second principal
surface 10b to the outside also has the output angle of 45 degrees.
Therefore, when focused on specific polarization components, the
reflectance R is the same. Further, the mirrors 21 and 31 are
assumed to have the reflectance of light of 100%.
[0043] When the dispersion compensating member 90 is not provided,
a light use efficiency I.sub.M1 of the incident light L.sub.0 that
reaches the detection unit 50 via the first branched light is
calculated by the following formula (1) as a product of the
reflectance (R) on the first principal surface 10a, the reflectance
(1.0) on the mirror 21, the transmittance (1-R) from the exterior
to the interior on the first principal surface 10a, and the
transmittance (1-R) from the interior to the exterior on the second
principal surface 10b, and thus, the light use efficiency is
R(1-R).sup.2.
[Formula 1]
[0044] I.sub.M1=R.times.1.0.times.(1-R).times.(1-R)=R(1-R).sup.2
(1)
[0045] A light use efficiency I.sub.M2 of the incident light
L.sub.0 that reaches the detection unit 50 via the second branched
light is calculated by the following formula (2) as a product of
the transmittance (1-R) from the exterior to the interior on the
first principal surface 10a, the transmittance (1-R) from the
interior to the exterior on the second principal surface 10b, the
reflectance (1.0) on the mirror 31, the transmittance (1-R) from
the exterior to the interior on the second principal surface 10b,
the reflectance (R) on the first principal surface 10a, and the
transmittance (1-R) from the interior to the exterior on the second
principal surface 10b, and thus, the light use efficiency is
R(1-R).sup.4.
[Formula 2]
[0046]
I.sub.M2=(1-R).times.(1-R).times.1.0.times.(1-R).times.R.times.(1--
R)=R(1-R).sup.4 (2)
[0047] An interference intensity peak I.sub.pp of the combined
light L.sub.3 is represented by the following formula (3).
[Formula 3]
[0048] I.sub.pp=2.times. {square root over (I.sub.M1I.sub.2)}=2
{square root over (R(1-R).sup.2R(1-R).sup.4)}=2R(1-R).sup.3 (3)
[0049] Assume that the branching-combining unit 10 is made of
silicon, the refractive index of silicon for the target wavelength
is 3.5, and the incident angle is 45 degrees, then the reflectance
R of the S-polarization components is about 43% and the reflectance
R of the P-polarization components is about 19%. Therefore, when
the incident light L.sub.0 includes only the S-polarization
components, the interference intensity peak I.sub.pp of the
combined light L.sub.3 is 15.9%. Further, when the incident light
L.sub.0 includes only the P-polarization components, the
interference intensity peak I.sub.pp of the combined light L.sub.3
is 20.2%. When the incident light L.sub.0 is in an arbitrary
polarization state, the interference intensity peak I.sub.pp of the
combined light L.sub.3 has a value between 15.9% and 20.2%. When
the incident light L.sub.0 is incoherent light, it is considered
that the polarization components of all directions are evenly
distributed in the incident light L.sub.0, and therefore, the
interference intensity peak I.sub.pp of the combined light L.sub.3
is 18.1% which is an average value of the S-polarization and the
P-polarization.
[0050] When the dispersion compensating member 90 is provided, and
when the reflectance of light is R.sub.e and the transmittance is
(1-R.sub.e) on the interface of the dispersion compensating member
90, the light use efficiency I.sub.M1 of the incident light L.sub.0
that reaches the detection unit 50 via the first branched light is
calculated by the following formula (.PHI.. The interference
intensity peak I.sub.pp of the combined light L.sub.3 is
represented by the following formula (5). That is, by providing the
dispersion compensating member 90, the interference intensity peak
I.sub.pp of the combined light L.sub.3 decreases by an amount of
the transmittance of light (1-R.sub.e) on the interface of the
dispersion compensating member 90.
[Formula 4]
[0051]
I.sub.M1=R.times.(1-R.sub.c).times.1.0.times.(1-R.sub.0).times.(1--
R).times.(1-R)=R(1-R).sup.2(1-R.sub.c).sup.2 (4)
[Formula 5]
[0052] I.sub.pp=2.times. {square root over (I.sub.M1I.sub.M2)}=2
{square root over
(R(1-R).sup.2(1-R.sub.c).sup.2R(1-R).sup.4)}=2R(1-R).sup.3(1-R.-
sub.c) (5)
[0053] When the dispersion compensating member 90 is provided, with
the branching-combining unit 10 being provided under the same
condition as above, and the light enters the dispersion
compensating member 90 vertically, then the reflectance of light
R.sub.c on the interface of the dispersion compensating member 90
is 30% regardless of the polarization state, and thus, the average
interference intensity peak I.sub.pp of the S-polarization and the
P-polarization of the combined light L.sub.3 is 12.7%. The
dispersion compensating member 90 is required in the usual FTIR,
and in this case, 12.7% is the maximum value of the interference
intensity peak I.sub.pp of the combined light L.sub.3.
[0054] If no excessive loss occurs in the configuration of the
optical interferometer 2A illustrated in FIG. 1, the light use
efficiency I.sub.M1 of the incident light L.sub.0 that reaches the
detection unit 50 via the first branched light is calculated by the
following formula (6). The same formula can be applied to calculate
the light use efficiency I.sub.M2 of the incident light L.sub.0
that reaches the detection unit 50 via the second branched light.
The interference intensity peak I.sub.pp of the combined light
L.sub.3 is represented by the following formula (7). Assume that no
excessive loss exists, the average interference intensity peak
I.sub.pp of the S-polarization and the P-polarization of the
combined light L.sub.3 is 39.9% (=(49.0+30.8)/2). Thus, the ratio
of the excessive loss is 68% (=(39.9-12.7)/39.9).
[Formula 6]
[0055] I.sub.M1=R.times.1.0.times.(1-R)=R(1-R) (6)
[Formula 7]
[0056] I.sub.pp=2= {square root over (I.sub.M1I.sub.M2)}=2 {square
root over (R(1-R)R(1-R))}=2R(1-R) (7)
[0057] Here, to decrease the excessive loss, AR (anti-reflection)
coating may be applied to the second principal surface 10b,
however, it is difficult to apply AR coating having a flat
wavelength characteristic over a wide wavelength range, so that the
wavelength range is limited. Further, when the optical
interferometer 2A is formed by the semiconductor process such as
MEMS, it is difficult to selectively apply AR coating on a specific
surface of the branching-combining unit 10, and if such case
happens, an inevitable loss may occur.
Second Comparative Example
[0058] FIG. 2 is a diagram illustrating a configuration of an
optical interferometer 2B of a second comparative example. The
optical interferometer 2B includes a branching-combining unit 10, a
mirror 21, a mirror 31, and a mirror 32.
[0059] The branching-combining unit 10 is made of, for example,
silicon and has a first principal surface 10a and a second
principal surface 10b which are parallel to each other. Incident
light L.sub.0 that enters from the outside to the first principal
surface 10a is partially reflected as first branched light
L.sub.11, and the rest of the incident light transmits into the
interior of the branching-combining unit 10 as second branched
light L.sub.21.
[0060] The first branched light L.sub.11 from the first principal
surface 10a is reflected by the mirror 21. The first branched light
L.sub.12 reflected by the mirror 21 enters the first principal
surface 10a, transmits into the interior of the branching-combining
unit 10, passes through the interior of the branching-combining
unit 10, and transmits through the second principal surface 10b to
be output to the outside.
[0061] The second branched light L.sub.21 from the first principal
surface 10a passes through the interior of the branching-combining
unit 10, transmits through the second principal surface 10b to be
output to the outside, is reflected by the mirror 31, and is
reflected again by the mirror 32. The second branched light
L.sub.22 reflected by the mirrors 31 and 32 enters the second
principal surface 10b and is reflected.
[0062] The first branched light L.sub.12 output to the outside on
the second principal surface 10b and the second branched light
L.sub.22 reflected on the second principal surface 10b are combined
to form combined light L.sub.3. The combined light L.sub.3 is
detected by a detection unit 50.
[0063] The optical interferometer 2B uses separate surfaces for the
branching surface (first principal surface 10a) to branch the
incident light L.sub.0 into the first branched light L.sub.11 and
the second branched light L.sub.21 and for the combining surface
(second principal surface 10b) to combine the first branched light
L.sub.12 and the second branched light L.sub.22 to form the
combined light L.sub.3.
[0064] For example, the position of the mirror 21 is fixed, and the
mirrors 31 and 32 are movable by a drive unit along the incident
direction of the second branched light L.sub.21. The drive unit can
also be configured by a MEMS-based component. Since the mirrors 31
and 32 are movable, the optical path difference between the first
branched light and the second branched light is adjustable.
[0065] The optical interferometer 2B of the second comparative
example allows the first branched light and the second branched
light to pass through the interior of the branching-combining unit
10 only once, thus preventing the problem of wavelength dispersion
without using the dispersion compensating member 90, which is
required in the optical interferometer 2A of the first comparative
example.
[0066] Further, the optical interferometer 2B of the second
comparative example decreases the excessive loss generated at the
interface between the branching-combining unit 10 and a surrounding
medium, when compared to the optical interferometer 2A of the first
comparative example. The excessive loss in the optical
interferometer 2B of the second comparative example is derived only
from the reflected light generated on the second principal surface
10b when the second branched light L.sub.21 transmits through the
second principal surface 10b to be output to the outside. The light
use efficiency I.sub.M1 of the incident light L.sub.0 that reach
the detection unit 50 via the first branched light, and the light
use efficiency I.sub.M2 of the incident light L.sub.0 that reach
the detection unit 50 via the second branched light, respectively,
are R(1-R).sup.2.
[0067] An interference intensity peak I.sub.pp of the combined
light L.sub.3 is represented by the following formula (8). Here,
the incident angle of the light entering the first principal
surface 10a or the second principal surface 10b from the outside is
set to 45 degrees. When the branching-combining unit 10 is made of
silicon, the average interference intensity peak I.sub.pp of the
S-polarization and the P-polarization of the combined light L.sub.3
is 26.4% (=(27.9+24.9)/2). Therefore, the ratio of the excessive
loss is 34% (=(39.9-26.4)/39.9). The excessive loss ratio in the
second comparative example is about half of 68% of the excessive
loss ratio in the first comparative example.
[Formula 8]
[0068] I.sub.pp=2.times. {square root over (I.sub.M1I.sub.M2)}=2
{square root over (R(1-R).sup.2R(1-R).sup.2)}=2R(1-R).sup.2 (8)
[0069] Further, the optical interferometer 2B has a problem
described below. In the optical interferometer 2B, the branching
surface (first principal surface 10a) is different from the
combining surface (second principal surface 10b), so that the first
branched light is reflected by the single mirror 21 and the second
branched light is reflected by the two mirrors 31 and 32 to combine
the first branched light and the second branched light on the
combining surface. By reflecting the second branched light by the
two mirrors 31 and 32, the reflecting position (combining position)
of the second branched light on the second principal surface 10b
differs from the output position of the second branched light from
the interior to the exterior on the second principal surface 10b,
and further, coincides with the output position of the first
branched light from the interior to the exterior on the second
principal surface 10b.
[0070] Here, consider light rays L.sub.0R and L.sub.0L that pass
through two different positions in the beam cross-section of the
incident light L.sub.0. The two light rays L.sub.0R and L.sub.0L of
the incident light L.sub.0 propagate through different paths within
a plane that is parallel to both the normal line of the first
principal surface 10a and the incident direction of the incident
light L.sub.0. Assume that, in the first branched light L.sub.11,
L.sub.12, a light ray L.sub.1R is derived from one light ray
L.sub.0R of the incident light L.sub.0, and a light ray L.sub.1L is
derived from the other light ray L.sub.0L of the incident light
L.sub.0. Assume that, in the second branched light L.sub.21,
L.sub.22, a light ray L.sub.2R is derived from one light ray
L.sub.0R of the incident light L.sub.0, and a light ray L.sub.2L is
derived from the other light ray L.sub.0L of the incident light
L.sub.0.
[0071] At this time, on the combining surface (second principal
surface 10b), the light ray L.sub.a of the first branched light
derived from the one light ray T.sub.0R of the incident light
L.sub.0 is combined with the light ray L.sub.21, of the second
branched light derived from the other light ray L.sub.0L of the
incident light L.sub.0. Further, the light ray L.sub.1L of the
first branched light derived from the other light ray L.sub.0L of
the incident light L.sub.0 is combined with the light ray L.sub.2R
of the second branched light derived from the one light ray
L.sub.0R of the incident light L.sub.0.
[0072] That is, when the light rays of the first branched light and
the second branched light reaching each position of the combining
surface (second principal surface 10b) are derived from different
light rays of the incident light, these light rays do not form
normal interference light even though the light rays are combined
to form the combined light. On the other hand, when the incident
light L.sub.0 is given as a light ray formed by expanding and
collimating a single light ray of, for example, a point light
source, the light rays of the first branched light and the second
branched light reaching each position may interfere with each
other, however, the quality of the interference signal may
decrease, because the optical path difference is generated by a
spatial distance between the light ray L.sub.1R and the light ray
L.sub.1L in the beam cross-section of the first branched light.
[0073] Normally, the optical interferometer is expected to have the
same optical path difference between the first branched light and
the second branched light that reach the respective positions as
the optical path difference adjusted by the movement of mirrors.
However, in the beam width range provided in the optical
interferometer 2B, the optical path difference changes between the
beam which is closer to the center and the beam which is closer to
the edge portion, whereby an observed interference signal is
averaged and weakened as a whole.
[0074] An optical interferometer used in FTIR uses light having a
large beam diameter in order to increase a parallel nature of the
propagating light. As a result, the light includes many beams
having different optical path differences, and thus the optical
interferometer 2B that uses the combined light beams at different
positions as the interference signal has an intrinsic problem with
respect to interference.
[0075] Further, the following problem would occur when the size of
the optical interferometer is decreased. The conventional infrared
spectroscopic devices, such as FTIR, are large and less portable,
however, recently, a small, lightweight, and highly portable
interferometer is desired to respond to the need to perform
infrared spectroscopy in the field where a measurement sample is
obtained. To satisfy this requirement, Patent Document 1 discloses
the invention of the MEMS-based optical interferometer. However,
decreasing the size of the MEMS-based optical interferometer would
decrease the interference efficiency in the optical interferometer
and cause deterioration of visibility by stray light.
[0076] FIG. 3 and FIG. 4 show interferograms when white light
enters the optical interferometer. The horizontal axis represents
the difference of the optical path lengths of the first branched
light and the second branched light from the branching surface to
the combining surface. The vertical axis represents intensity
(interferogram) of the combined light L.sub.3 (optical interference
signal). Since the incident light is white light, the strongest
interference occurs when the optical path difference is 0, and the
interference becomes weaker as the absolute value of the optical
path difference is larger. Assume that an average value of the
optical interference signal is m, and a difference between the
maximal value and the minimal value of the optical interference
signal is .delta., then the visibility is represented by .delta./(2
m).
[0077] The visibility approaches 1 in an ideal optical
interferometer in which no stray light exists (FIG. 3). As the
stray light increases, the average value m of the optical
interference signal increases and the difference .delta. between
the maximal value and the minimal value of the optical interference
signal decreases, so that the value of the visibility decreases
(FIG. 4). A small optical interferometer is not able to completely
collimate the incident light (make the incident light be parallel)
which is to be measured, and further, cannot have a sufficiently
large branching surface and combining surface, causing a loss by
the beam expansion of the light, and further, a phenomenon such as
the light that does not contribute to interference reaching the
detection unit occurs, thus decreasing the value of the
visibility.
[0078] For example, when the light to be measured is guided through
an optical fiber having a core diameter of 400 .mu.m and NA of 0.2,
and output from an output end of the optical fiber into the optical
interferometer, the following result is obtained. The beam
expansion of the light in the optical interferometer decreases
resolution, and the degree of the decrease is a relative accuracy
of wavenumber of about 2 (1-cos(NA)). For example, when it is
desired to measure the wavenumber 5000 cm.sup.-1 (wavelength 2
.mu.m) at the resolution of not more than 10 cm.sup.-1 (4 .mu.m), a
relative accuracy of wavenumber needs to be 1/500 and NA needs to
be 0.045 by calculating 2 (1-cos(NA))=1/500.
[0079] When a collimator lens capable of achieving this NA is
inserted, the NA 0.2 beam is converted to the NA 0.045 beam, and
the beam size after conversion is 1800 .mu.m which is 0.2/0.045
times the beam size before conversion. In practice, the completely
collimated light is not achievable, so that the optical
interferometer needs to be configured with a sufficiently
large-sized optical system with respect to the beam size.
[0080] However, in the case of MEMS, for example, such a large
optical system cannot be used. For example, the size limit is about
1 mm, so that most of the beam would be lost. The loss is partially
propagated outside of the optical interferometer, and the rest of
the loss is received by the detection unit as stray light. The
received stray light is not the interference light, and increases
the average value m of the above-described interferogram. Further,
even when the beam can be propagated in the optical system of the
optical interferometer, some beams that cannot be the interference
light exist due to beam expansion, and such beams also contribute
to increase the average value m of the interferogram. The degree of
increase of the average value in of the interferogram depends on
the size of the optical system and the optical interferometer. The
average value m of the interferogram sometimes becomes several to
ten times the difference .delta. between the maximal value and the
minimal value of the optical interference signal.
[0081] To remove the influence of the stray light, a signal output
from the detection unit needs to be processed. When, however, the
combined light (interference light) is weak, an SIN of the signal
output from the detection unit would decrease. When the intensity
of the interference light is small, it is possible to increase the
value of the output signal from the detection unit by increasing a
gain of an amplifier in the detection unit, however, when the
average value is large, the dynamic range of the amplifier would be
fully used for amplification of the average value signal. As
described above, the inefficient optical interferometer decreases
the signal intensity of the intrinsic optical interference signal,
and in addition, causes the increase of the average value till the
dynamic range of the first-stage amplifier is fully used, thus
further decreasing the S/N when the signal is obtained.
First Embodiment
[0082] FIG. 5 is a diagram illustrating a configuration of an
optical interferometer 1A of a first embodiment. The optical
interferometer 1A includes a branching-combining unit 10, a first
optical system 20, a second optical system 30, and a drive unit 40.
Preferably, these components of the optical interferometer 1A are
configured by MEMS-based components.
[0083] The branching-combining unit 10 is made of a transparent
member of a semiconductor, such as silicon, and has a branching
surface 11, an incident surface 12, a first output surface 13, a
combining surface 14, and a second output surface 15 on interfaces
between the interior and the exterior of the transparent
member.
[0084] The branching-combining unit 10, on the branching surface
11, partially reflects incident light L.sub.0 that enters from the
outside and outputs as first branched light L.sub.11, and transmits
the rest of the incident light into the interior as second branched
light L.sub.21. The branching-combining unit 10, on the incident
surface 12, transmits the first branched light L.sub.12 that enters
from the branching surface 11 via the first optical system 20 into
the interior. The branching-combining unit 10, on the first output
surface 13, outputs the second branched light L.sub.21 that reaches
from the branching surface 11 through the interior to the
outside.
[0085] The branching-combining unit 10, on the combining surface
14, partially combines the first branched light L.sub.12 that
reaches from the incident surface 12 through the interior, and the
second branched light L.sub.22 that enters from the first output
surface 13 via the second optical system 30 to be output to the
outside as first combined light L.sub.3, and combines the rest of
the first branched light and the second branched light to be
propagated into the interior as second combined light L.sub.4. The
branching-combining unit 10, on the second output surface 15,
outputs partial light L.sub.41 of the second combined light L.sub.4
that reaches from the combining surface 14 through the interior to
the outside.
[0086] The first optical system 20 reflects the first branched
light L.sub.11 output from the branching surface 11 by mirrors 21
and 22, and directs the reflected first branched light L.sub.12 to
the incident surface 12. The second optical system 30 reflects the
second branched light L.sub.71 output from the first output surface
13 by mirrors 31 and 32, and directs the reflected second branched
light L.sub.22 to the combining surface 14. The drive unit 40 moves
any of the mirrors of the first optical system 20 or the second
optical system 30 to adjust an optical path difference between the
first branched light and the second branched light from the
branching surface 11 to the combining surface 14.
[0087] The directions of the branching surface 11, the incident
surface 12, the first output surface 13, and the combining surface
14 of the branching-combining unit 10 and the light incident
positions and the incident angles on the respective surfaces are
set appropriately according to the refractive indexes of the
branching-combining unit 10 and the surrounding medium, so that the
first branched light and the second branched light are combined
coaxially on the combining surface 14 and output as the combined
light L.sub.3 and the combined light L.sub.4 at the same output
angle. The first branched light and the second branched light have
the same optical path length in the branching-combining unit 10.
This eliminates the problem of wavelength dispersion and decreases
wavelength dependency of the optical path difference between the
first branched light and the second branched light.
[0088] The branching surface 11 and the combining surface 14 are
provided separately. The branching surface 11, the incident surface
12, and the second output surface 15 may not be parallel to each
other, may be parallel to each other, and may be provided on a
common plane. An incident region of the incident light L.sub.0 on
the branching surface 11 and an incident region of the first
branched light L.sub.12 on the incident surface 12 may be different
or may coincide with each other partially or entirely. The first
output surface 13 and the combining surface 14 may not be parallel
to each other, may be parallel to each other, and may be provided
on a common plane. An output region of the second branched light
L.sub.21 on the first output surface 13 and an output region of the
combined light on the combining surface 14 may be different or may
coincide with each other partially or entirely.
[0089] The incident light L.sub.0 that enters the branching surface
11 from the outside is partially reflected as the first branched
light L.sub.11, and the rest of the incident light transmits into
the interior of the branching-combining unit 10 as the second
branched light L.sub.21.
[0090] The first branched light L.sub.11 from the branching surface
11 is reflected by the mirrors 21 and 22 of the first optical
system 20. The reflected first branched light L.sub.12 enters the
incident surface 12, transmits into the interior of the
branching-combining unit 10, passes through the interior of the
branching-combining unit 10, and reaches the combining surface
14.
[0091] The second branched light L.sub.21 from the branching
surface 11 passes through the interior of the branching-combining
unit 10, transmits through the first output surface 13 to be output
to the outside, and is reflected by the mirrors 31 and 32 of the
second optical system 30. The reflected second branched light
L.sub.22 reaches the combining surface 14.
[0092] A component of the first branched light L.sub.12 that
reaches the combining surface 14 and transmits through the
combining surface 14, and a component of the second combined light
L.sub.22 that reaches the combining surface 14 and is reflected by
the combining surface 14 are combined and output to the outside as
the first combined light L.sub.3. The first combined light L.sub.3
output to the outside from the combining surface 14 is received by
a first light receiving element 51.
[0093] A component of the first branched light L.sub.12 that
reaches the combining surface 14 and is reflected by the combining
surface 14, and a component of the second combined light L.sub.22
that reaches the combining surface 14 and transmits through the
combining surface 14 are combined and propagated into the interior
as the second combined light L.sub.4. The second combined light
L.sub.4 reaches the second output surface 15, and the partial light
L.sub.41 is output to the outside from the second output surface
15. The second combined light L.sub.41 output to the outside from
the second output surface 15 is received by a second light
receiving element 52.
[0094] The first light receiving element 51, the second light
receiving element 52, and a current-voltage conversion unit 54
constitute a detection unit that detects an interference signal.
The first light receiving element 51 receives the first combined
light L.sub.3 output to the outside from the combining surface 14,
and outputs a first current signal I.sub.1 according to the amount
of received light. The second light receiving element 52 receives
the second combined light L.sub.41 output to the outside from the
second output surface 15, and outputs a second current signal
I.sub.2 according to the amount of received light. The
current-voltage conversion unit 54 outputs a voltage signal
according to a value (I.sub.1-I.sub.2) obtained by subtracting the
second current signal I.sub.2 from the first current signal I. The
detection unit will be described later in detail.
[0095] The optical interferometer 1A of the present embodiment has
an even number of mirrors in total in the first optical system 20
and the second optical system 30, and therefore can branch the
light ray at each position in the beam cross-section of the
incident light L.sub.0 on the branching surface 11, and then
combine the light rays at a common position in the beam
cross-section of the combined light L.sub.3 and the combined light
L.sub.4 on the combining surface 14, and thus, the interference
efficiency can be improved compared to the configuration of the
second comparative example.
[0096] Further, the optical interferometer 1A of the present
embodiment differs from the configuration of the second comparative
example, in that, in addition to the first light receiving element
51 that receives the first combined light L.sub.3 output to the
outside from the combining surface 14, the second light receiving
element 52 receives the second combined light L.sub.41 output to
the outside from the second output surface 15. Thus, the optical
interferometer 1A of the present embodiment can decrease the ratio
of the excessive loss when compared to the configuration of the
second comparative example.
[0097] The excessive loss in the present embodiment is calculated
as follows. The above formula (8) represents the interference
intensity peak I.sub.pp of the first combined light L.sub.3 which
is output from the combining surface 14 and received by the first
light receiving element 51. The following formula (11) represents
the interference intensity peak I.sub.pp of the second combined
light L.sub.41 which is output from the second output surface 15
and received by the second light receiving element 52, obtained by
the light use efficiency I.sub.M1 (the following formula (9)) of
the incident light L.sub.0 that reaches the second light receiving
element 52 via the first branched light, and the light use
efficiency I.sub.M2 (the following formula (10)) of the incident
light L.sub.0 that reaches the second light receiving element 52
via the second branched light.
[0098] When the interference signals of both the first combined
light L.sub.3 and the second combined light L.sub.41 can be used,
the total interference intensity peak I.sub.ppT is represented by
the following formula (12) by adding the formula (8) and the
formula (11).
[Formula 9]
[0099]
I.sub.M1=R.times.1.0.times.(1-R).times.R.times.(1-R)=R.sup.2(1-R).-
sup.2 (9)
[Formula 10]
[0100]
I.sub.M2=(1-R).times.(1-R).times.1.0.times.(1-R).times.(1-R)=(1-R)-
.sup.4 (10)
[Formula 11]
[0101] I.sub.pp=2.times. {square root over (I.sub.M119 I.sub.M2)}=2
{square root over (R.sup.2(1-R).sup.2(1-R).sup.4)}=2R(1-R).sup.3
(11)
[Formula 12]
[0102] I.sub.ppT=2R(1-R).sup.2+2R(1-R).sup.3=2R(1-R).sup.2(2-R)
(12)
[0103] Assume that the incident angle is 45 degrees for all light
beams that enters respective surfaces from the outside, and the
branching-combining unit 10 is made of silicon, then the average
interference intensity peak I.sub.ppT of the S-polarization and the
P-polarization of the combined light L.sub.3 and the combined light
L.sub.41 is 44.5% (=(43.9+45.1)/2). Thus, the ratio of the
excessive loss is -11.5% (=(39.9-44.5)/39.9). That is, the present
embodiment can achieve an efficiency beyond the limit in principle
in the configuration of the second comparative example. This is
achieved by using the partial light L.sub.41 of the second combined
light L.sub.4 which is intrinsically inevitable loss in principle
on the combining surface 14 as an effective interference
signal.
[0104] This is described by referring to an optical interferometer
2C that has a general configuration for performing both branching
and combining with a single beam splitter as illustrated in FIG. 6.
FIG. 6 is a diagram illustrating a configuration of the optical
interferometer 2C of a third comparative example.
[0105] In the optical interferometer 2C, incident light L.sub.0
enters a beam splitter 110 and is branched by the beam splitter 110
to first branched light L.sub.1 of a reflected component, and
second branched light L.sub.2 of a transmitted component. The first
branched light L.sub.1 is reflected by a mirror 120 to enter the
beam splitter 110 again, and the second branched light L.sub.2 is
reflected by a mirror 130 to enter the beam splitter 110 again. A
component of the first branched light L.sub.1 that enters the beam
splitter 110 from the mirror 120 and transmits through the beam
splitter 110, and a component of the second branched light L.sub.2
that enters the beam splitter 110 from the mirror 130 and is
reflected by the beam splitter 110 are combined to form combined
light L.sub.3.
[0106] Assume that the reflectance of light of the beam splitter
110 is R, the transmittance of light of the beam splitter 110 is
1-R. At this time, the interference intensity peak I.sub.pp of the
combined light L.sub.3 is represented by the following formula
(13). The interference intensity peak I.sub.pp is a quadratic
function of a variable R, and reaches the maximum value 0.5 (50%)
at R=0.5. That is, in the optical interferometer 2C, an ideal
optical branching ratio of the beam splitter 110 is 1:1 to reach
the maximum interference intensity peak I.sub.pp.
[Formula 13]
[0107] I.sub.pp=2.times. {square root over
(R(1-R).times.R(1-R))}=2R(1-R) (13)
[0108] In the configuration of the optical interferometer 2C
illustrated in FIG. 6, a component of the first branched light
L.sub.1 that enters the beam splitter 110 from the mirror 120 and
is reflected by the beam splitter 110, and a component of the
second branched light L.sub.2 that enters the beam splitter 110
from the mirror 130 and transmits through the beam splitter 110 are
combined to form combined light L.sub.4. To detect the combined
light L.sub.4, consider an optical interferometer 2D having a
configuration as illustrated in FIG. 7. FIG. 7 is a diagram
illustrating a configuration of the optical interferometer 2D of a
fourth comparative example.
[0109] The optical interferometer 2D of the fourth comparative
example illustrated in FIG. 7 includes a beam splitter 111 in
addition to the configuration of the optical interferometer 2C of
the third comparative example illustrated in FIG. 6. The beam
splitter 111 is inserted on the optical path of the incident light
L.sub.0 to detect the combined light L.sub.4. By inserting the beam
splitter 111, the loss of the incident light L.sub.0 occurs. As a
result, assume that the optical branching ratio of each of the beam
splitters 110 and 111 is 1:1, the interference intensity peak
I.sub.pp of the total of the combined light L.sub.3 and the
combined light L.sub.4 is 37.5%.
[0110] This means that the optical interferometer 2C that has a
general configuration and performs both branching and combining
with a single beam splitter is the ideal interferometer capable of
achieving the maximum efficiency of 50%, and this maximum
efficiency 50% is regarded as the limit efficiency in principle.
The reason why the configuration of the optical interferometer 1A
of the present embodiment illustrated in FIG. 5 can overcome such a
limit efficiency in principle is that the branching-combining unit
10 includes the branching surface 11 and the combining surface 14
separately, with the branching surface 11 and the combining surface
14 disposed at a certain interval from each other, so that the
second output surface 15 from which the second combined light
L.sub.41 is output, and the branching surface 11 that receives the
incident light L.sub.0 can be provided separately.
[0111] Here, the ideal reflectance of light R of the beam splitter
110 is 50% in the configuration of the optical interferometer 2C of
the third comparative example illustrated in FIG. 6. However, the
ideal reflectance of light R in the optical interferometer 1A of
the present embodiment illustrated in FIG. 5 at respective surfaces
is not 50%. The interference intensity peak I.sub.ppT of the total
of the combined light L.sub.3 and the combined light L.sub.41 in
the optical interferometer 1A of the present embodiment is a
quartic function of the variable R, as represented by the above
formula (12). In the range of 0<R<1, the maximum value is
49.9% at R=30%. Ideally, R=30% in the optical interferometer 1A of
the present embodiment. Further, at R=50%, the interference
intensity peak I.sub.ppT is 37.5%.
[0112] When the branching-combining unit 10 is made of silicon, and
the light enters each surface vertically, the ideal beam splitter
is provided with the reflectance of 30%. In the actual design,
however, the vertical incidence is not possible, and therefore, the
average interference intensity peak is compared by considering the
S-polarization components and the P-polarization components, as
described above.
[0113] FIG. 8 is a graph illustrating a relationship between the
refractive index of a material of the branching-combining unit 10
and the average interference intensity peak. Calculation is based
on the light incident angle of 45 degrees when the light enters the
branching-combining unit 10 from the outside. From the calculation
result, it is found that the average interference intensity peak
reaches the maximum value 44% at the refractive index 3.5. Since
the refractive index of silicon is 3.5 in the near-infrared region,
silicon is probably the ideal material for forming the
branching-combining unit 10 in the optical interferometer 1A of the
present embodiment for branching and combining using the difference
in refractive index between air and the material of the
branching-combining unit 10.
[0114] Next, a method of detecting the interference signal of the
combined light L.sub.3 and the combined light L.sub.41 in the
optical interferometer 1A of the first embodiment is described.
First to third detecting methods will be described
sequentially.
[0115] A first detecting method of receiving light with a single
photodetector by optically combining the first combined light
L.sub.3 and the second combined light L.sub.41 in some way is
considered. Further, since the white light is simply formed by
linearly adding light beams having continuous wavelengths, light
having one wavelength alone is considered below. Assume that co
represents an angular frequency of light and t represents time
variable.
[0116] An electric field E.sub.3 of the first combined light
L.sub.3 output from the combining surface 14 is represented as a
sum of the electric field a.sub.13 sin (.omega.t+.PHI..sub.13) of
the first branched light reaching the combining surface 14 and the
electric field a.sub.23 sin (.omega.t+.PHI..sub.23) of the second
branched light reaching the combining surface 14, as represented by
the following formula (14) and formula (15).
[ Formula 14 ] E 3 = a 13 sin ( .omega. t + .phi. 13 ) + a 23 sin (
.omega. t + .phi. 23 ) = a 3 sin ( .omega. t + .phi. 3 ) ( 14 ) [
Formula 15 ] a 3 = a 13 2 + a 23 2 + 2 a 13 a 23 cos ( .phi. 13 -
.phi. 23 ) .phi. 3 = tan - 1 [ a 13 sin ( .phi. 13 ) + a 23 sin (
.phi. 23 ) a 13 cos ( .phi. 13 ) + a 23 cos ( .phi. 23 ) ] ( 15 )
##EQU00001##
[0117] When a square law detector such as a photodiode receives the
first combined light L.sub.3, the signal output from the square law
detector represents a temporal average value of a square of the
electric field E.sub.3 of the first combined light L.sub.3 (i.e., a
square of an amplitude a.sub.3 of the electric field), and depends
on a phase difference (.PHI..sub.13-.PHI..sub.23). This is the
fundamental principle for obtaining a signal change according to
the optical path difference using the optical interferometer. The
phase difference (.PHI..sub.13-.PHI..sub.23) in the formula of the
amplitude a.sub.3 of the electric field E.sub.3 of the first
combined light L.sub.3 corresponds to the optical path difference
between the first branched light and the second branched light set
by the drive unit, and therefore, has no relationship with an
initial phase. The phase .PHI..sub.3 in the sin function relating
to the temporal change of the electric field E.sub.3 of the first
combined light L.sub.3 depends on both the optical path difference
and the initial phase.
[0118] An electric field E.sub.4 of the second combined light
L.sub.41 output from the second output surface 15 is represented as
a sum of the electric field a.sub.14 sin (.omega.t+.PHI..sub.14) of
the first branched light reaching the second output surface 15 and
the electric field a.sub.24 sin (.omega.t+.PHI..sub.24) of the
second branched light reaching the second output surface 15, as
represented by the following formula (16) and formula (17).
[ Formula 16 ] ##EQU00002## E 4 = a 14 sin ( .omega. t + .phi. 14 )
+ a 24 sin ( .omega. t + .phi. 24 ) = a 4 sin ( .omega. t + .phi. 4
) [ Formula 17 ] ( 16 ) a 4 = a 14 2 + a 24 2 + 2 a 14 a 24 cos (
.phi. 14 - .phi. 24 ) .phi. 4 = tan - 1 [ a 14 sin ( .phi. 14 ) + a
24 sin ( .phi. 24 ) a 14 cos ( .phi. 14 ) + a 24 cos ( .phi. 24 ) ]
( 17 ) ##EQU00002.2##
[0119] Assume that a phase change when the second combined light
propagates from the combining surface 14 to the second output
surface 15 is represented by .PHI..sub..omega.. The phase change
.PHI..sub..omega. depends on the angular frequency .omega.. Phases
.PHI..sub.13, .PHI..sub.23, .PHI..sub.14, and .PHI..sub.24 are
related as represented by the following formula (18). Therefore,
the phase difference (.PHI..sub.14-.PHI..sub.24) in the formula of
the amplitude a.sub.4 of the electric field E.sub.1 of the second
combined light is equal to the phase difference
.PHI..sub.13-.PHI..sub.23) in the formula of the amplitude a.sub.3
of the electric field E.sub.3 of the first combined light.
[Formula 18]
[0120] .PHI..sub.14=.PHI..sub..omega.+.PHI..sub.13
.PHI..sub.24=.PHI..sub..omega.+.PHI..sub.23 (18)
[0121] For simplification, consider the case where the optical path
lengths of the first branched light and the second branched light
are set equal by the drive unit, that is, the case where both the
phase difference (.PHI..sub.13-.PHI..sub.23) and the phase
difference (.PHI..sub.14-.PHI..sub.24) are 0. At this time, the
following formula (19) is satisfied.
[Formula 19]
[0122] .PHI..sub.3=.PHI..sub.13
.PHI..sub.4=.PHI..sub..omega.+.PHI..sub.13 (19)
[0123] An electric field E.sub.T of the total combined light
obtained by combining the first combined light L.sub.3 and the
second combined light L.sub.41 is represented as a sum of the
electric field E.sub.3 of the first combined light L.sub.3 and the
electric field E.sub.4 of the second combined light L.sub.41 as
represented by the following formula (20) and formula (21).
[ Formula 20 ] ##EQU00003## E T = E 3 + E 4 = a 3 sin ( .omega. t +
.phi. 3 ) + a 4 sin ( .omega. t + .phi. 4 ) = a 3 sin ( .omega. t +
.phi. 13 ) + a 4 sin ( .omega. t + .phi. .omega. + .phi. 13 ) = a T
sin ( .omega. t + .phi. T ) [ Formula 21 ] ( 20 ) a T = a 3 2 + a 4
2 + 2 a 3 a 4 cos ( .phi. .omega. ) .phi. T = tan - 1 [ a 3 sin (
.phi. 13 ) + a 4 sin ( .phi. .omega. + .phi. 13 ) a 3 cos ( .phi.
13 ) + a 4 cos ( .phi. .omega. + .phi. 13 ) ] ( 21 )
##EQU00003.2##
[0124] A signal output from the photodetector receiving the total
combined light differs for each wavelength, as affected by the
phase .PHI..sub..omega. that differs for each wavelength. This
means that the interference signal changes due to a factor other
than the optical path difference between the first branched light
and the second branched light, so that the optical interferometer
does not function properly.
[0125] A second detecting method of detecting the first combined
light L.sub.3 and the second combined light L.sub.41 separately
with different photodetectors and adding voltage signals after
square-law detection by respective photodetectors is considered. In
this method, no influence of the phase .PHI..sub..omega. appears,
so that adding the voltage signals as the interference signal
functions properly. Generally, however, the photodetector includes
an amplifier for amplifying the signal to obtain a voltage level
appropriate for processing the signal after output. When a
photovoltaic type light receiving element is used, an overall S/N
is determined according to a noise of the amplifier in the first
stage. When the first combined light L.sub.3 and the second
combined light L.sub.41 are detected by different photodetectors
and the signals are amplified, the overall noise becomes twice as
large as the noise of the first-stage amplifier of each
photodetector. Thus, the signal level increases by adding the
voltage signals, however, the S/N is not improved. In the present
embodiment, the efficiency of the first combined light L.sub.3 is
26.4% and the efficiency of the second combined light L.sub.41 is
17.8%, and thus, the signal amount is not simply doubled regardless
of the addition, and the S/N decreases.
[0126] A third detecting method of detecting the first combined
light L.sub.3 and the second combined light L.sub.41 with different
light receiving elements, adding/subtracting the current signals
output from the light receiving elements, and converting the result
to the voltage signal is considered. In this method, the S/N
decreasing problem described above can be avoided. The third
detecting method is described below, and the configuration of the
detection unit 50 according to this method is described as
well.
[0127] Electromagnetic waves including light sometimes change their
phases when being reflected by an interface between two media
having different refractive indexes. An amount of phase change at
the reflection of light on the interface depends on the refractive
index of each medium, the incident angle of light to the interface,
and the polarization state of light. Assume that each medium is
made of a dielectric material and an absorption coefficient can be
ignored, then the phase change occurs at the reflection of light on
the interface when the S-polarization light enters the interface
from the medium having a low refractive index, and when the
P-polarization light enters the interface from the medium having a
high refractive index at an incident angle not exceeding a
Brewster's angle, and in both cases, a phase change amount is
.pi..
[0128] By considering such a characteristic, the phase change
amount at the reflection of light in the configuration of the
optical interferometer 1A illustrated in FIG. 5 is considered.
Assume that the phase of the first branched light L.sub.12 and the
second branched light L.sub.22 that enter the combining surface 14
is a reference phase, a phase change .pi. occurs when the
S-polarization component of the second branched light L.sub.22 is
reflected on the combining surface 14 to form the first combined
light L.sub.3, and a phase change .pi. occurs when the
P-polarization component of the first branched light L.sub.12 is
reflected on the combining surface 14 to form the second combined
light L.sub.41.
[0129] Thus, the second combined light L.sub.41 output from the
second output surface 15 has a phase difference .pi. with respect
to the first combined light L.sub.3 output from the combining
surface 14 for the S-polarization component and the P-polarization
component. That is, the first combined light L.sub.3 and the second
combined light L.sub.41 form the interference signals having
opposite signs. In addition to the generation of the phase change
.pi. at the reflection of the S-polarization component of the
second branched light L.sub.22 on the combining surface 14 as
described above, the phase change .pi. is generated in the
S-polarization component of the first branched light L.sub.11 at
the branching of the incident light L.sub.0 on the branching
surface 11, and therefore, the S-polarization components of the
first branched light L.sub.12 and the second branched light
L.sub.22 at the combining have the same phase. Further, the
P-polarization components of the first branched light L.sub.12 and
the second branched light L.sub.22 at the combining have also the
same phase. As a result, the S-polarization components and the
P-polarization components have the interference signal of the same
sign.
[0130] Here, in practice, when the mirrors in the first optical
system 20 and the second optical system 30 are metallic mirrors,
the phase change occurs at the reflection of light by the metallic
mirror. When the first optical system 20 and the second optical
system 30 reflect light by the mirrors having a common
configuration, the same phase change occurs for both the
S-polarization components and the P-polarization components, so
that, as a result, the interference signals of the S-polarization
components and the P-polarization components have the same
sign.
[0131] For the first combined light L.sub.3 and the second combined
light L.sub.41, the interference signals have opposite signs over
the entire wavelengths. FIG. 9 shows interferograms of the combined
light L.sub.3 and the combined light L.sub.41. When the optical
path difference is 0, the interference signal of the first combined
light L.sub.3 is at the maximum value, and the interference signal
of the second combined light L.sub.41 is at the minimum value. One
of the interferogram of the first combined light L.sub.3 and the
interferogram of the second combined light L.sub.41 corresponds to
the interferogram obtained by reversing the other interferogram
upside down about the average value and correcting the amplitude
according to the efficiency.
[0132] Here, in FIG. 9, the interferograms of the first combined
light L.sub.3 and the second combined light L.sub.41 are shown with
different average values. The average value of the interferogram
depends on the. configuration of the optical interferometer. For
example, when the average value of the interferogram increases
because more background light enters the photodetector than
non-interference light from the optical interferometer, the
difference of the average values of the interferograms between the
first combined light L.sub.3 and the second combined light L.sub.41
is not so large. In contrast, when the average value of the
interferogram increases because more non-interference light from
the optical interferometer enters the photodetector than the
background light, the average value is determined at a ratio
similar to the efficiency of the interference signal at each
interferogram of the first combined light L.sub.3 and the second
combined light L.sub.41.
[0133] Thus, by obtaining the difference between the interferograms
of the first combined light L.sub.3 and the second combined light
L.sub.41, the average value decreases and the interference signal
increases. FIG. 10 shows the difference between the interferograms
of the combined light L.sub.3 and the combined light L.sub.41. By
obtaining the difference between them, interference signal may
partially be a negative value which, however, causes no problem on
a circuit of the detection unit 50 illustrated in FIG. 11.
[0134] FIG. 11 is a circuit diagram of the detection unit 50 of the
optical interferometer 1A of the first embodiment. The detection
unit 50 includes the first light receiving element 51, the second
light receiving element 52, and the current-voltage conversion unit
54. The current-voltage conversion unit 54 includes an amplifier 55
and a feedback resistor 56. The feedback resistor 56 is provided
between an inverted input terminal and an output terminal Vo of the
amplifier 55. The first light receiving element 51 and the second
light receiving element 52 are photovoltaic type elements such as
photodiodes. The first light receiving element 51 and the second
light receiving element 52 are connected in series between a first
reference potential terminal V1 and a second reference potential
terminal V2 to which an inverse voltage is applied. V1 is a
positive potential and V2 is a negative potential. A connection
point between the first light receiving element 51 and the second
light receiving element 52 is connected to the inverted input
terminal of the amplifier 55. The non-inverted input terminal of
the amplifier 55 is connected to a ground potential terminal. The
inverted input terminal, which is in an imaginary short
relationship with the non-inverted input terminal of the amplifier
55, is also at the ground potential.
[0135] The first light receiving element 51 receives the first
combined light L.sub.3 output to the outside from the combining
surface 14, and outputs a first current signal I.sub.1 according to
the amount of received light. The second light receiving element 52
receives the second combined light L.sub.41 output to the outside
from the second output surface 15, and outputs a second current
signal I.sub.2 according to the amount of received light. A current
(I.sub.1-I.sub.2) which is a difference between the first current
signal I.sub.1 and the second current signal I.sub.2 flows in the
feedback resistor 56, and a voltage value according to a product of
the current value of the difference and the resistance value of the
feedback resistor 56 appears at the output terminal Vo. That is,
the current-voltage conversion unit 54 can output a voltage signal
according to a value (I.sub.1-I.sub.2) obtained by subtracting the
second current signal I.sub.2 from the first current signal I.sub.1
from the output terminal Vo.
[0136] The voltage signal output from the output terminal Vo
represents the difference between the interferograms of the first
combined light L.sub.3 and the second combined light L.sub.41, to
decrease the average value of the interferogram and emphasize the
interference signal. The voltage signal output from the output
terminal Vo has a noise determined only by the single amplifier 55,
so that the S/N is improved. Further, the decrease of the average
value of the interferogram allows the gain of the amplifier 55 to
be appropriately set in consideration of the magnitude of the
interference signal.
Second Embodiment
[0137] FIG. 12 is a diagram illustrating a configuration of an
optical interferometer 1B of a second embodiment. The optical
interferometer 1B includes a branching-combining unit 10, a first
optical system 20, a second optical system 30, and a drive unit 40.
Preferably, these components of the optical interferometer 1B are
configured by MEMS-based components.
[0138] The branching-combining unit 10 is made of a transparent
member of a semiconductor, such as silicon, and has a branching
surface 11, an incident surface 12, a first output surface 13, a
combining surface 14, a second output surface 15, and a third
output surface 16 on interfaces between the interior and the
exterior of the transparent member.
[0139] The optical interferometer 1B of the second embodiment
illustrated in FIG. 12 differs from the configuration of the
optical interferometer 1A of the first embodiment illustrated in
FIG. 5 in that the branching-combining unit 10 further includes the
third output surface 16. Further, the optical interferometer 1B of
the second embodiment differs from the configuration of the optical
interferometer 1A of the first embodiment in that the
interferometer not only detects the partial light L.sub.41 of the
second combined light L.sub.4 transmitted through the second output
surface 15 and output to the outside, but also detects, by a third
light receiving element 53, the rest of the light L.sub.42 of the
second combined light L.sub.4 reflected by the second output
surface 15, transmitted through the third output surface 16, and
output to the outside.
[0140] The number of reflections of the second combined light
L.sub.42 output from the third output surface 16 is only one time
larger than that of the second combined light L.sub.41 output from
the second output surface 15. Therefore, the interference intensity
peak I.sub.pp2 of the second combined light L.sub.42, which is
output from the third output surface 16 and received by the third
light receiving element 53, is R times the interference intensity
peak I.sub.pp1 of the second combined light L.sub.41, which is
output from the second output surface 15 and received by the second
light receiving element 52, as represented by the following formula
(22).
[Formula 22]
[0141] I.sub.pp2=I.sub.pp1.times.R=2R(1-R).sup.3R (22)
[0142] Assume that the incident angle is 45 degrees for all light
beams that enters respective surfaces from the outside, and the
branching-combining unit 10 is made of silicon, then the average
interference intensity peak of the S-polarization and the
P-polarization of the second combined light L.sub.42 output from
the third output surface 16 and received by the third light
receiving element 53 is 5.3%. The average interference intensity
peak of the P-polarization and the S-polarization of the total of
the combined light L.sub.3, the combined light L.sub.41, and the
combined light L.sub.42 is 49.5%, thus achieving an approximately
50% efficiency.
[0143] Here, the second combined light reflected by the third
output surface 16 may be output to the outside from another output
surface and received by another light receiving element, and when
the entire second combined light L.sub.4 is detected, an overall
efficiency of the second combined light L.sub.4 is 26.4%. A sum of
the efficiency 26.4% of the first combined light L.sub.3 and the
overall efficiency 26.4% of the second combined light L.sub.4 is
52.8%, and this is improved from the efficiency of 49.5% of the
total of the combined light L.sub.3, the combined light L.sub.41,
and the combined light L.sub.42 in the configuration of the optical
interferometer 1B of FIG. 12, however, the degree of improvement is
small. When considering the small improvement and more complicated
configuration, it is not advantageous to output the second combined
light reflected by the third output surface 16 from another output
surface to the outside and receive the output light by another
light receiving element.
[0144] FIG. 13 is a circuit diagram of a detection unit 50 of the
optical interferometer 1B of the second embodiment. The detection
unit 50 includes the first light receiving element 51, the second
light receiving element 52, the third light receiving element 53,
and the current-voltage conversion unit 54. The detection unit 50
of the second embodiment illustrated in FIG. 13 differs from the
configuration of the detection unit 50 of the first embodiment
illustrated in FIG. 11 in that the third light receiving element 53
is further included. The third light receiving element 53 is also a
photovoltaic type element such as a photodiode.
[0145] The phase change at the reflection of the second combined
light L.sub.42 which is output from the third output surface 16 is
the same as the phase change at the reflection of the second
combined light L.sub.41 which is output from the second output
surface 15. Therefore, the third light receiving element 53
receiving the second combined light L.sub.42 output from the third
output surface 16 is provided in parallel to the second light
receiving element 52 that receives the second combined light
L.sub.41 output from the second output surface 15. The third light
receiving element 53 receives the second combined light L.sub.42
output to the outside from the third output surface 16, and outputs
the third current signal I.sub.3 according to the amount of
received light. The current-voltage conversion unit 54 outputs a
voltage signal according to a value (I.sub.1-I.sub.2-I.sub.3)
obtained by subtracting the second current signal I.sub.2 and the
third current signal I.sub.3 from the first current signal I.sub.1
from the output terminal Vo.
[0146] Here, in the optical interferometer 1B of the second
embodiment, it is necessary to pay more attention when the
background light that enters the photodetector is larger than the
non-interference light from the optical interferometer to increase
the average value of the interferogram. In this case, the average
values of the interferograms of the respective combined light beams
are about the same, so that the average value component of a single
interferogram appears in the value (I.sub.1-I.sub.2-I.sub.3)
obtained by subtracting the second current signal I.sub.2 and the
third current signal I.sub.3 from the first current signal I.sub.1.
As a result, the addition of the third light receiving element 53
may sometimes narrows the dynamic range. It is necessary,
therefore, to determine whether the addition of the third light
receiving element 53 is effective or not by considering if the S/N
is finally improved.
Third Embodiment
[0147] FIG. 14 is a diagram illustrating a configuration of an
optical interferometer 1C of a third embodiment. The optical
interferometer 1C includes a branching-combining unit 10, a first
optical system 20, a second optical system 30, and a drive unit 40.
Preferably, these components of the optical interferometer 1C are
configured by MEMS-based components.
[0148] The branching-combining unit 10 is made of a transparent
member of a semiconductor, such as silicon, and has a branching
surface 11, an incident surface 12, a first output surface 13, a
combining surface 14, a second output surface 15, and a total
reflection surface 17 on interfaces between the interior and the
exterior of the transparent member.
[0149] The optical interferometer 1C of the third embodiment
illustrated in FIG. 14 differs from the configuration of the
optical interferometer 1A of the first embodiment illustrated in
FIG. 5 in that the branching-combining unit 10 further includes the
total reflection surface 17. The branching-combining unit 10, on
the total reflection surface 17, totally reflects the second
combined light L.sub.4 that reaches from the combining surface 14
through the interior and directs the light to the second output
surface 15.
[0150] In the third embodiment, the direction of the total
reflection surface 17 is set such that the total reflection surface
17 can totally reflect the second combined light L.sub.4, that is,
the incident angle of the second combined light L.sub.4 to the
total reflection surface 17 is equal to or larger than a critical
angle. Further, the direction of the second output surface 15 is
set such that the second combined light L.sub.4 totally reflected
by the total reflection surface 17 can be output to the outside
from the second output surface 15, that is, the incident angle of
the second combined light L.sub.4 to the second output surface 15
is less than the critical angle.
[0151] In the first embodiment, the branching surface 11 to which
the incident light L.sub.0 enters, and the second output surface 15
from which the second combined light L.sub.41 is output are
provided on the same side of the branching-combining unit 10, and
this sometimes limits the arrangement of the optical systems for
the incident light L.sub.0 and the second combined light L.sub.41.
In contrast, the third embodiment includes the branching surface 11
on one side of the branching-combining unit 10, and the combining
surface 14 and the second output surface 15 on the other side of
the branching-combining unit 10, thus facilitating the arrangement
of the optical systems for the incident light L.sub.0 and the
combined light L.sub.3 and the combined light L.sub.41.
[0152] The optical interferometer according to the present
invention is not limited to the embodiments and configuration
examples described above, and various modifications can be
made.
[0153] An optical interferometer of the above-described embodiment
includes a branching-combining unit, a first optical system, a
second optical system, and a drive unit. The branching-combining
unit includes a branching surface, an incident surface, a first
output surface, a combining surface, and a second output surface on
an interface between the interior and the exterior of a transparent
member, the branching surface and the combining surface are
provided separately, the branching surface partially reflects
incident light entering from the outside and outputs as first
branched light, and transmits the rest of the incident light into
the interior as second branched light, the incident surface
transmits the first branched light entering from the branching
surface via the first optical system into the interior, the first
output surface outputs the second branched light reaching from the
branching surface through the interior to the outside, the
combining surface partially combines the first branched light
reaching from the incident surface through the interior, and the
second branched light entering from the first output surface via
the second optical system to be output to the outside as first
combined light, and combines the rest of the first branched light
and the second branched light to be propagated into the interior as
second combined light, and the second output surface partially
outputs the second combined light reaching from the combining
surface through the interior to the outside. The first optical
system reflects the first branched light output from the branching
surface by a mirror, and directs the light to the incident surface.
The second optical system reflects the second branched light output
from the first output surface by a mirror, and directs the light to
the combining surface. The drive unit moves the mirror of the first
optical system or the second optical system to adjust an optical
path difference between the first branched light and the second
branched light from the branching surface to the combining
surface.
[0154] Preferably, the optical interferometer of the
above-described configuration further includes a first light
receiving element receiving the first combined light output from
the combining surface and outputting a first current signal I.sub.1
according to an amount of the received light, a second light
receiving element receiving the second combined light partially
output from the second output surface and outputting a second
current signal I.sub.2 according to an amount of the received
light, and a current-voltage conversion unit outputting a voltage
signal according to a value (I.sub.1-I.sub.2) obtained by
subtracting the second current signal I.sub.2 from the first
current signal I.sub.1.
[0155] Preferably, in the optical interferometer of the
above-described configuration, the branching-combining unit further
includes a third output surface on the interface between the
interior and the exterior of the transparent member, and the third
output surface outputs the rest of the second combined light
reflected by the second output surface and reaching through the
interior to the outside.
[0156] Further, preferably, in this case, the optical
interferometer of the above-described configuration further
includes a first light receiving element receiving the first
combined light output from the combining surface and outputting a
first current signal I.sub.1 according to an amount of the received
light, a second light receiving element receiving the second
combined light partially output from the second output surface and
outputting a second current signal I.sub.2 according to an amount
of the received light, a third light receiving element receiving
the rest of the second combined light output from the third output
surface and outputting a third current signal I.sub.3 according to
an amount of the received light, and a current-voltage conversion
unit outputting a voltage signal according to a value
(I.sub.1-I.sub.2-I.sub.3) obtained by subtracting the second
current signal I.sub.2 and the third current signal I.sub.3 from
the first current signal I.sub.1.
[0157] Preferably, in the optical interferometer of the
above-described configuration, the branching-combining unit further
includes a total reflection surface on the interface between the
interior and the exterior of the transparent member, and the total
reflection surface totally reflects the second combined light
reaching from the combining surface through the interior and
directs the light to the second output surface.
[0158] Preferably, in the optical interferometer of the
above-described configuration, the first optical system and the
second optical system include an even number of mirrors in total,
and the optical interferometer branches a light ray at each
position in a beam cross-section of the incident light on the
branching surface, and then combines the light rays at a common
position in a beam cross-section of the first combined light and
the second combined light on the combining surface.
[0159] Preferably, in the optical interferometer of the
above-described configuration, the branching-combining unit, the
first optical system, the second optical system, and the drive unit
are MEMS (Micro Electro-Mechanical System)-based components.
Further, preferably, in the optical interferometer of the
above-described configuration, the branching-combining unit is made
of silicon.
[0160] Preferably, in the optical interferometer of the
above-described configuration, the first branched light and the
second branched light have the same optical path length in the
branching-combining unit.
INDUSTRIAL APPLICABILITY
[0161] The present invention can be used as an optical
interferometer capable of decreasing a ratio of excessive loss.
REFERENCE SIGNS LIST
[0162] 1A-1C--optical interferometer, 10--branching-combining unit,
11--branching surface, 12--incident surface, 13--first output
surface, 14--combining surface, 15--second output surface,
16--third output surface, 17--total reflection surface, 20--first
optical system, 21, 22--mirror, 30--second optical system, 31,
32--mirror, 40--drive unit, 50--detection unit, 51--first light
receiving element, 52--second light receiving element, 53--third
light receiving element, 54--current-voltage conversion unit,
55--amplifier, 56--feedback resistor, 90--dispersion compensating
member, L.sub.0--incident light, L.sub.11, L.sub.12--first branched
light, L.sub.21, L.sub.22--second branched light, L.sub.3--first
combined light, L.sub.4, L.sub.41, L.sub.42--second combined
light.
* * * * *